There is an ever present demand for IC fabricating fabs to increase their yield and productivity, while reducing cost. This demand has led to an ever constant attempt to rework ICs which are defective in order to salvage time and money expended in fabricating correct portions of the IC.
In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit (IC) fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, leaving a photoresist mask corresponding to the desired pattern to be formed on the silicon wafer.
In the process discussed above it is important that the patterning of the photoresist yield not only the desired pattern size but a correctly aligned pattern. In some instances the patterned photoresist can be incorrectly patterned due to, for example, line-width variations such as out-of-specification line widths, mispatterning or overlay/pattern mismatch with a lower layer.
Reworking or re-patterning a photoresist of an IC device is economically desirable, as compared to scrapping the wafer, when there is at least one correctly constructed lower layer (e.g., a silicon wafer) already formed beneath a photoresist layer. However, the process of stripping the photoresist pattern layer or portion thereof may result in damage to or change a top monolayer of oxide portion of an anti-reflective coating (ARC) which lies on top of a polysilicon layer. A change in the monolayer may result in exposure dose change as well as interaction of the deep UV photoresist with the ARC material.
More particularly, a top monolayer of a silicon oxy-nitride ARC is converted into an oxide by N.sub.2 O plasma to prevent nitrogen contact with the photoresist layer formed over the ARC. Nitrogen contact with the photoresist may result in undesirable footing problems (non-uniform structure). Thus, the oxide monolayer serves as a barrier between the photoresist and nitrogen portion of the ARC. Such a monolayer works fine during an intial photoresist application and photolithographic process. However, if the photoresist needs to be reworked the plasma and chemical process employed in the rework to strip the photoresist may result in removal of the oxide monolayer. Consequently, nitrogen bonds of the silicon oxy-nitride may react with a newly applied chemically amplified deep UV photoresist resulting in footing problems in the new photoresist layer.
Thus, there is a need in the art for a method which permits easy reworking of an incorrectly formed photoresist layer while preventing nitrogen contact between an ARC and newly applied photoresist layer.